Kaolin deposits in Egypt occur
in three main areas namely, Sinai, Red Sea and Aswan. In Sinai, kaolins occur in the Nubia
succession at MussabaSalama,
El-Tih and Farsh El-Ghozlan with total reserves of about 100 million metric tones. In the Red Sea area kaolin deposits located at the Hafayir and Abou-Darag
localities, near Suez. The Hafayir kaolin is 12.5 m
thick and confined to the sandy clayey calcareous Miocene sediments. The Abou-Darag kaolin bed is 17.8 m thick in the Nubia sandstone. The WadiKalabsha kaolin, holding around 17 million metric tones, is situated at about 150 km of west of Aswan.

Mineralogical and geochemical
investigations indicated that these deposits are composed of kaolinite with traces of quartz and of detrital
origin. Iron and titanium occur as ilmenite and anatase in very fine grains as rhombs and net structure. No
iron and titanium were found in the structure of the kaolinite.
The study suggests the authigenic origin of ilmenite and rutile that are
probably formed during the diagenesis of these
deposits.

Kaolin is produced from company's mines in Eltieh area through underground mines and open cast area,
the production capacity is about 72000 tons/ year. Used for several industries
like ceramics ,white ware refractory industry , paper
productions, cement.

The production of Kaolin from
Sinai is up to 100,000 MTPA. It is considered as one of the best Kaolin
deposits in the Middle East. The most important markets for Kaolin are
refractory, cement, ceramic and paper industries. In addition to the
considerable portion needed for local demands of Kaolin, that ore is export to
Spain, Turkey and some Arabian countries, mainly for the white cement industry.

_________________

Froth Flotation

The Froth "Flotation
System" as shown in above Figure includes many inter-related components as
follows:

Froth flotation is a
good example of an engineering “system”, in that the various important
parameters are highly inter-related, as shown in above Figure. It is therefore
important to take all ofthese
factors into account in froth flotation operations. Changes in the settings of
one factor (such as feed rate) will automatically cause or demand changes in
other parts of the system (such as flotation rate, particle size recovery, air
flow, pulp density, etc.) As a result, it is difficult to study the effects of
any single factor in isolation, and compensation effects within the system can
keep process changes from producing the expected effects (Klimpel,
1995). This makes it difficult to develop predictive models for froth flotation,
although work is being done to develop simple models that can predict the
performance of the circuit from easily- measurable parameters such as solids
recovery and tailings solid content (Rao et al.,
1995).

Flotation Cell Technolgy

Conventional Cells

Conventional flotation cells
consist of a tank with an agitator designed to disperse air into the slurry, as
shown schematically in Figure below. These are relatively simple machines, with
ample opportunity for particles to be carried into the froth along with the
water making up the bubble films (entrainment), or for hydrophobic particles to
break free from the froth and be removed along with the hydrophilic particles.
It is therefore common for conventional flotation cells to be assembled in a
multi-stage circuit, with “rougher”, “cleaner”, and “scavenger” cells, which
can be arranged in configurations such as the one shown in Figure below.

Flotation Columns

Flotation columns provide a
means for improving the effectiveness of froth flotation (Eberts,
1986). A column essentially performs as if it were a multistage flotation
circuit arranged vertically (Degner and Sabey, 1985), with slurry flowing downward while the air
bubbles travel upward, producing a countercurrent flow. The first flotation
machine design to use a countercurrent flow of slurry and air was developed by
Town and Flynn in 1919. It was not until the work of Boutin
and Tremblay in the early 1960’s that a new generation of countercurrent
columns was developed that ultimately became industrially successful
(Rubinstein, 1995).

A typical flotation column is
shown in Figure below. The basic principle of column flotation is the use of
countercurrent flow of air bubbles and solid particles. This is achieved by
injecting air at the base of the column, and feed near the midpoint. The
particles then sink through a rising swarm of air bubbles.

Countercurrent flow is
accentuated in most columns by the addition of washwater
at the top of the column, which forces all of the water which entered with the
feed downward, to the tailings outlet. This flow pattern is in direct contrast
to that found in conventional cells, where both the air and the solid particles
are driven in the same direction. The result is that columns provide improved
hydrodynamic conditions for flotation, and thus produce a cleaner product while
maintaining high recovery and low power consumption. The performance
differences between columns and conventional cells may best be described in
terms of the following factors: collection zone size, particle/bubble contact
efficiency, and fines entrainment (Kawatra and Eisele, 1987).

Columns exhibit higher
particle/bubble contact efficiency than conventional machines, due to the
particles colliding with the bubbles head-on. As a result, the energy intensity
needed to promote contact is less, and so power consumption is reduced.

A second beneficial effect in
certain types of flotation columns is the reduction of bubble diameter (Yoon
and Luttrell, 1986). As bubble diameter is reduced, the flotation rate of both
the coarser and finer particles is improved. Coarse particles can attach to
more than one bubble if the bubbles are small, and therefore the chances of the
particle being torn loose and sinking again is reduced. For fine particles, the
probability of collision with the bubble is improved if the bubble is small, as
then the hydrodynamic forces tending to sweep the particle away from a
collision are reduced. The reduction of bubble diameter has the added benefit
of increasing the available bubble surface area for the same amount of injected
air. It is therefore desirable to produce bubbles as fine as possible.

Carrier flotation is
considered as unconventional flocculation and flotation techniques compared by
the conventional flocculation and flotation techniques. Carrier flotation is
known as ultra-fine flotation. In flotation, extremely fine paerticles
that are difficult to attach to air bubbles adhere to coarse carriers, and
float with them . This technique was originally used
to remove anatase (TiO2) from kaolin using
coarse calcite as a carrier, and has been developed into autogenous
carrier flotation, where fine particles are carried on coarse particles of the
same ore. This is a great advance in flotation tehnology.
Since it has advantage over conventional flotation techniques in speration result and reagent consumption, it has broad
prospects for industrial use.

The concept of using particles
of high floatability (usually coarser size) as a carrier to carry particles of
less floatability (usually ultrafine in size) has been tested on a variety of
ores and coals. In this technique, the particles to be floated coat the carrier
material and the coated particles are then floated. The first commercial use of
this process was in the purification of kaolin at Minerals and Chemicals
Philipp's plant-Georgia USA (Seeton, 1961). This
process was originally developed to remove titaniferrous
impurities from kaolin clay using 60 μm
limestone particles as the carrier mineral.

This paper aims to upgrade an
Egyptian kaolin preconcentrate sample by reducing its
anatase impurities (TiO2) through the
conventional froth flotation technique. The flotation tests were carried out on
a kaolin sample of about 80 wt.% below 1.59μm. Sodium
silicate and oleic acid - (An oily liquid occurring in animal and vegetable oils and
used in making soap; chemical formula:C18H34O2) – were used as a depressant
and a collector respectively. The different operating (chemical and mechanical)
parameters were studied.

The results showed that the
mechanical factors just as the speed and time of conditioning played the vital
role in determining the efficiency of the flotation process for such ultrafine
particles. For example, changing the dosage of collector or the pH have no
influence on the grade of the produced concentrates if the flotation process
was performed at low conditioning speed or at small conditioning time. However,
the reverse trend was noticed when the flotation tests were carried out at high
conditioning speed and after long conditioning time.

At the optimum operating
conditions a concentrate of about 0.63% TiO2 with a%age removal of 77.5 was obtained from a feed containing
1.52% TiO2.

Introduction:

Kaolin

Kaolin is used in
a multiplicity of industries because of its unique physical and chemical
properties. Shape, particle size, color, softness,andnonabrasiveness are
physical properties that are especially important. Anatase
(TiO2) represents one of the major discoloring impurities in kaolin
that reduces its brightness. Kaolin with brightness greater than 90 is produced
by incorporating either magnetic separation, froth
flotation, and/or selective flocculation. Each of the individual processes has
its own merit and limitations and therefore, a combination of two or three of
these techniques is common in the industry to make the
best use of mined kaolin. At least one plant is making use of a froth flotation
process for kaolin that does not involve the use of carrier mineral. Although
carrier flotation can work well with a given crude
kaolin, it may not respond with another deposit.

--------------

The following is
added to the paper by Atef Helal

The first
commercial use of carrier flotation process was originally developed at Georgia
USA (Seeton 1961) to upgrade kaolin. Titaniferrous impurities were removed with 60-μm
limestone particles as the carrier mineral. In addition, it is not always
necessary to externally add particles because the coarser particles originally
present may act as a carrier. When coarse particles that may act as carrier
particles are inherently present, the addition of carrier mineral may or may
not improve the process.

A comparison of
conventional froth flotation, carrier flotation, and column flotation
techniques at their optimum conditions for separation of titaniferrous
impurities from Egyptian kaolin ores is presented in above Table. Flotation was performed
using oleic acid as a collector for the colored impurities, and sodium silicate
as a depressant for kaolin. These results showed that conventional froth
flotation gave a kaolin concentrate (84.75 wt %)
containing 0.68% TiO2 and 0.56% Fe2O3 with a
degree of whiteness 78 from a feed assaying 1.52% TiO2, 0.37% Fe2O3,
and whiteness 56. Carrier flotation provided a concentrate of similar grade but
of a higher degree of whiteness (90). Column flotation produced a concentrate
with a lower TiO2 content (0.38%), Fe2O3%
decreased to 0.49% and the whiteness reached 91.5. its higher value.

End of the
addition – Atef Helal

---------------------

The Egyptian kaolin is hard
and massive. Attrition scrubbing followed by screening and hydrocycloning
separation processes can remove the majority of its associated free quartz and
some iron oxides. However, these preconcentrates
still have high TiO2 content and consequently are not used in paper
coating or fine ceramics. This paper aims to upgrade such preconcentrates
by reducing there anatase mineral (TiO2)
through the conventional froth flotation technique. The different operating
parameters were studied.

Experimental And Materials:

A representative preconcentrate kaolin sample of El-Tih
locality, Sinai , was used as a flotation feed , its
chemical analysis is : Al2O3 (36.40%) and SiO2
(47.58%). This means that the concentration of the kaolinite
mineral is high ( abt. 94.64%) while that of free
silica is low (abt.4.69%). The sample contains also a relatively higher content
of TiO2 (1.52%) than that required of paper coating and fine
ceramics. The loss on ignition was 13.58%. On the other hand
, the size analysis showed the very fine grade size distribution of the
sample , where it contained about 79.92 wt.% below 1.95 μm.
The fraction above 44.2 μm represented only 5.23
wt.% of the sample.

Laboratory grade oleic acid
and technical grade sodium silicate were used as a collector and a dispersing
agent respectively. Analytical grade sulphuric acid
and sodium carbonate were used as pH regulators.

Flotation tests were carried
out in a "Denver D12" flotation cell with 1.5 lit. stainless
steel container. In each test, about 300 gm of kaolin sample was conditioned at
pH 10.5 (if otherwise not mentioned), adjusted by sodium carbonate, in presence
of one kg/ton sodium silicate at impeller speed 2500 rpm (if otherwise not
mentioned). The collector was added as ammonium salt of oleic acid by its
agitating with ammonium hydroxide (4:1 wt/wt ratio) in 100 ml water volume for
15 min. This prepared solution was added to the pulp and conditioned

for a certain period at solid/liquid ratio of 50%, and then
flotation was carried out at an impeller speed of 2000 rpm (if otherwise not
mentioned). The floated an non-floated fractions were
dried, weighed and amalyzed for TiO2. The
whiteness degree was measured by DR LANGE whiteness tester.

Results and Discussions :

Definitions

In the evaluation of the
flotation results the following definitions are used :
1- Retention Ratio (R-R) for TiO2 . This value varies from 1.0 for
no separation to zero for complete separation. 2- Coefficient of separation
(C.S). 3- Amount of TiO2 removed (Wang and Somasundarm,1980).

Effect of
conditioning speed of kaolin with reagents

Figure 1 depicts the effect of
changing the speed of conditioning of kaolin with reagents. It is clear that
the conditioning speed plays a vital role in determining the efficiency of the
flotation process. The TiO2 content of the concentrates was gradually
decreased from about 1.52% in the feed to 0.93% with a % removal of about 61 and
a R.R. of 0.62 by merely increasing the conditioning speed to 2500 rpm. The C.S
was significantly improved about 10 times , Fig 1.

Wang and Somasundarm
(1980)discussed
the reasons of improving the separation at high conditioning speed. They
attributed such improvement to 3 reasons: improving aggregation
, increasing the rate of oleate adsorption ,
and increasing the temperature of the pulp due to the high speed.

However, the level of the
conditioning speed used in these flotation tests (1500-2500) are relatively
high that adsorption can not be expected to be
controlled by diffusion to any measurable extent. The beneficial effect of
increased conditioning speed on flotation can not
therefore be accounted for in the present case by a diffusion enhanced process.

Meanwhile, the measurements of
temperature during the flotation tests indicated that temperature was slightly
increased by about 4oC (from 20oC at the beginning of the
conditioning process to 24oC at the end of the experiment). Such
small increase in temperature as a result of increasing the conditioning speed
suggests that it can only produce minor improvement in flotation.

From the above discussions , it could be suggested
that the improvement of the kaolin grade at higher conditioning speed may be
related to the aggregation between various particles of anatase,i.e.
formation of anatase-anatase aggregates.

Effect of
conditioning time of kaolin with oleic acid

Figure 2 depicts the effect of
changing the conditioning time of kaolin with oleic acid. It is clear that such
time plays a very important role in determining the grade of obtained
concentrate. A significant drop in the TiO2 content from 1.35 to
0.86% with a % removal of about 61.5 was noticed by increasing the conditioning
time from 5 to 20 min. At the latter conditions the R.R. and C.S. values
reached to 0.56 and 0.20 respectively whereas the whiteness improved to about
69. Further increase in the conditioning time caused a slight improvement in
the grade and whiteness degree. However the lowest TiO2 content
(0.75%) with the highest % removal ( approx. 67.5%)
and whiteness (79) was obtained at the longest conditioning time of 35 min.

At such conditions the R.R and
C.S. reached their best values (0.49 and 0.32 respectively), Figure 2.

Trahar and Warren (1967) mentioned
that ultrafine paticles float more slowly than those
of intermediate sizes. Moreover, each of the ultrafine fractions may be further
subdivided into slow and fast floating components. The decrease in the rate of
flotation of fast component appears to be the main reason for the slow overall
flotation rate of the ultrafines. In the mean time,
Woodburn et al (1971) argued that the rate of flotation was equal to the
product of three factors: the rate of collision between particles and bubbles;
the probability of adhesion and the probability that the adhering particles
would not be detached subsequently.

Thus the flotation rate will
depend, among other things, on the particle size.The
lower the particle size the slower the flotation rate. The effect of particle
size will, then, be
a predominant phenomenon in the flotation of kaolin since the flotation feed
contains about 79.92 wt% below 1.95 μm. This
means that the probability of collision between particles and bubbles will be minimum and consequently the flotation process may become a
function of time.

Thus, at longer conditioning
time (e.g. 35 min) the probability of formation of anatase
–anatase aggregates may increase which in turn will
improve both their collision rate with air bubbles and the probability of
adhesion. This may explain the significant improvement in the efficiency of the
flotation of such fine particles at longer conditioning time, For this reason, the longest conditioning time (35 min.) of
pulp with oleic acid will be used in the next tests.

Figure 3 depicts
the effect of changing flotation speed on the performance of the flotation process. The
results indicate that the flotation speed plays, also, an important role in
determining the efficiency of the flotation process. Increasing the flotation
speed within the range 1200-2000 rpm caused a successive improvement in the
grade of the obtained concentrates where the TiO2 content was
decreased from about 1.38% at the lowest flotation speed (1200 rpm) to 0.79%
only at 2000 rpm. At this range of speed the R.R. and C.S. values reached their
best values (0.52 and 0.30 respectively) whereas the % TiO2 removed
attained its maximum value (66.45%). The degree of whiteness showed a
progressive improvement, from 62 to 72, with increasing the flotation speed
from 1200 to 2000 rpm . On the contrary, the results
indicated that when the flotation speed increased over 2000 rpm (e.g. 2250 rpm)
a significant reduction in the flotation efficiency was noticed. At the latter
case, the TiO2 content was increased to about 0.95% with a
significant drop in its % removal to 53.75. The whiteness and C.S. grcreased to 68 and 0.18 respectively while the R.R.
increased to about 0.63, Figure 3.

The flotation rate depends, as
mentioned before, upon three factors : collision,
adhesion and detachement. These factors can be
affected by the flotation speed. An increase in the impeller speed, whatever the
size of the bubble or the particle, results in a greater detachment force
between a bubble and a particle caught in the turbulent field around the
impeller. Elswhere in the stirred cell where the
turbulence levels are less than in the impeller region ,
it is likely that an increase in the impeller speed will lead to an increase in
the rate of capture of the particles by bubbles. The overall flotation rate is
thus a balance between the competing mechanisms.

Thus with increasing the
flotation speed from 1200 to 2000 rpm the rate of collision between particles
and bubbles, and their subsequent adhesion would increase. This may lead to
each significant improvement in the grade of kaolin and its whiteness. However,
at the highest impeller speed (2250 rpm) the rate of detachment would be dreater than that of collision and adhesion leading to poor
flotation of anatase as shown in Figure 3.

Effect of Changing pH

The effect of changing pH medium was studied
over a relatively wide range (2-10.5) at different speeds of conditioningand flotation, the results of which are shown
in Figure 4. The results showed, again, the importance of optimizing the
chemical factors after the mechanical ones. For example, changing the pH of the
medium over the all pH region (2-10.5) at lower conditioning and flotation
speeds (2000 amd 1500 rpm respectively) did not show
any variation in the TiO2 content (1.36 %) with a very small removal
percent (abt. 10 to 16%). This made the C.S. very poor (abt. 0.02 to 0.09) allover the pH region (results are not shown).

However, repeating the same
flotation tests at the optimum speeds of conditioning (2500 rpm) and flotation
(2000 rpm) showed the dependence of the efficiency of the process on the pH of
the medium, Figure 4. The TiO2 content was not affected within pH
3-6 afterwhich the anatase
impurities started to gradually decrease with increasing pH till 10.5 where the
lowest TiO2 (0.69%) with a %age removal of about 64.11 was obtained.
At the latter case , the R.R. reached its minimum
value (0.45) whereas the C.S. was significantly improved to 0.34. Also, the
whiteness improved to 79, Figure 4.

The variation of pH affects
the sign and magnitude of the charge of kaolinite and
anatase. It is known that the face surface charge on
the kaolinite remains negative over a wide pH range
(3-11) (Grim,1953).Abdel Rahman (1996) has found that
pure kaolinite and anatase
remain negative at pH's above their point of zero charge PZC (3.23 and 3.65
respectively) and both minerals are completely dispersed in the pH range 6-10.

It is clear that the effect of
pH can be understood by considering the degree of dispersion, distribution of
collector species and surface charge charahteristics
of the anatase. When the flotation tests were
performed at the optimum speeds of conditioning and flotation (Figure 4), the
results showed a dependence upon the pH. In the
latter case, the TiO2 content was not changed within the acidic pH
region (2-6) probably due to the lack of good dispersion for the pulp.
Meanwhile, the flotation process was significantly improved within the alkaline
pH where successive reduction in the TiO2 content was noticed with
raising the pH till 10.5 at which the lowest anatase
content was obtained. Also, the R.R. reached its minimum value (0.45) while C.S
improved to 0.35. Such improvement at pH 10.5 could be related to the complete
dispersion of the kaolin pulp which, in turn, may lead to better distribution
for oleate species on the anatase
particles.

Effect of oleic
acid dosage

The effect of changing the
dosage of oleic acid was studied at two different levels of conditions. First
at lower conditioning speed (2000 rpm) and time (15 min.) and the second series
at the optimum conditioning speed (2500 rpm) and time (35 min), the results of
the latter are shown in Figure 5.

The results indicate that the
dosage of oleic acid had a major effect if conditioning was performed at lower
speed (2000 rpm) and for small time (15 min.), regardless of the dosage added.
The TiO2 content was not changed with increasing the dosage of oleic
acid up to 2.0 kg/ton above which a minor change to 1.21 % was noticed at a
dosage of 3.5 kg/ton. At such conditions, the R.R. was reduced to 0.80 while
the C.S was only 0.11. It seems that the flotation process is not selective at
a relatively weak conditioning level. This could be related to the weak
dispersion of the kaolin pulp which may adversely affect the adsorption of
oleic acid on the anatase particles.

If the degree of dispersion
plays such vital role in determining the selectivity of flotation
, the results should be improved while repeating the experiments at the
optimum conditioning speed (2500 rpm) and time (35 min). This has been
confirmed in the results shown in Figure 5. A large drop in the TiO2
content (from 1.52% in the feed to 0.75%) was noticed while adding only 1.0
kg/ton of oleic acid. At the latter case, about 60% of TiO2 was
removed while the R.R. was reduced to 0.49 with a significant improvement for
the C.S. value (0.32). Such reduction in TiO2 content was not
obtained at the weak level of conditioning even after adding 3.5 kg/ton of
oleic acid. The results in Figure 5 also showed that further addition of oleic
acid to 2.0 kg/ton decreased again the TiO2 % to 0.61 with a %age
removal of about 79. At such conditions, the whiteness was improved to 81 while
the R.R. and C.S. values were nearly constant. Thr
results showed that a dosage of 1 kg/ton of oleic acid was enough to be used in
the next experiments. It seems that performing the flotation process at the
optimum conditioning speed and for enough time can greatly help in improving
the selectivity. Such high conditioning speed can facilitate the aggregation
process of anatase-anatase aggregates which at enough
conditioning time enhance their collision and subsequent adhesion with the oleate species thereby improving their flotation as shown
in Figure 5.

Effect of dosage
of sodium silicate

Figure 6 shows the effect of
changing the dosage of sodium silicate on the efficiency of the flotation
process. The results indicated that the TiO2 content was decreased
from 0.92% to 0.75% with increasing the dosage of sodium silicate from 0.5 to
1.0 kg/ton. At such dosage about 60% of TiO2 was removed which
improved the whiteness to about 78. Also, the C.S. and R.R. were at their
optimum values (0.32 and 0.49 respectively). The results also showed that the
higher dosage can deteriorate the selectivity of the flotation process. It
seems that 1.0 kg/ton of sodium silicate is the optimum dosage.

The importance of the
flotation performance with increasing the dosage of sodium silicate up to 1.0
kg/ton could be related to its dual role in the kaolin pulp, i.e. as a
depressant for the kaolinite mineral and at the same
time as a good dispersing agent for the kaolin slurry. So, with increasing the
amount of sodium silicate up to its optimum dosage the silicate ions can adsorb,oftenly physically at the positive edges of the kaolinite mineral leading to a corresponding reduction in
the possibility edge-to-face flocculation process and consequently enhances the
dispersion of kaolin. This makes the flotation process more selective by
floating only the anatase impurities. Such adsorption
is expected to increase at the higher dosage of sodium silicate leading to an
excessive increase in the negative charge of both the kaolinite
and the anatase minerals thereby hindering the
adsorption of the collector (oleate ions) on the
surface of the latter and consequently deteriorates the selectivity of the
flotation process as shown in Figure 6.

Effect of
changing the conditioning temperature

Figure 7 shows the effect of
changing the conditioning temperature. These results showed a significant
reduction in the TiO2 content from 0.75% to 0.63% with a
considerable improvement in the whiteness of the obtained concentrates from 74
to 79, with raising the conditioning temperature from 20 to 40oC.
This was, also, reflected in an improvement in the R.R and C.S. values as well
as the %age TiO2 removed. It is clear that the increase in
conditioning temperature is beneficial for improving the grade of kaolin.

The variation in temperature
can affect the system through increasing the collector adsorption on anatase, solubility of collector, solubility of minerals
and viscosity of the liquid medium. The adsorption ………

Meanwhile, flotation can be
expected to improve to some extent with increase in conditioning temperature
owing to the decrease in viscosity of water also .
Based on the above discussions the increased collection adsorption on anatase mineral is proposed to be mainly responsible for
such improvement in flotation with increasing the conditioning temperature.

Conclusion

1.In the
froth flotation of Kaolin preconcentrate sample
(97.92 wt.% below 1.95 μm)
the mechanical parameters (such as conditioning speed and time and flotation
speed) should be optimized before that of the chemical ones. Otherwise no
change in the performance of flotation can be noticed. This because kaolin
needs a high conditioning speed, a long conditioning time and a suitable
flotation speed. Meanwhile, the degree of dispersion of the pulp plays a vital
role in determining the selectivity of anatase flotation.
Such degree of dispersion can greatly improved by performing the tests in an
alkaline medium (pH 10.5) with the optimum dosage of sodium silicate. The
selectivity of the process can be further improved at conditioning temperature
of about 40oC.

2.At the
optimum conditions for froth flotation of anatase
impurities from the kaolin, a concentrate containing only 0.63% TiO2
with a whiteness of 79 could be obtained from a feed assaying 1.52% TiO2
with a whiteness of 56.

Note : The rest of
present paper discusses " Charachteristics of
tailings and tailing management", and not available electronically.